1. Field of the Invention
The present invention relates to a transformer, especially to a planar transformer and a balanced-to-unbalanced transformer (BALUN).
2. Description of Related Art
A transformer is usually employed in an RF (radio frequency) IC for the purpose of impedance matching for received signals. A transformer inside an IC typically includes planar coils so that impedance matching can be realized by the coupling effects between coils with appropriate turns ratio. In addition to impedance matching, the RF IC in a signal receiving end may also have to convert signals between a common mode and a differential mode. A BALUN is usually utilized in this kind of signal conversion. A BALUN, which is also made of coils in an IC, is one of many applications of a transformer. Therefore, for both transformers and BALUNs, a good design for coils in terms of excellent coupling effects, high quality factor Q and reduced magnetic radiation becomes more and more significant.